1. Field of the Invention
The present invention relates to a semiconductor device mainly composed of MOSFETs (metal-oxide-semiconductor field effect transistors), and more specifically to a semiconductor device including electrically writable non-volatile semiconductor memory such as EEPROM (electrically erasable program read only memory).
2. Description of Related Art
Conventionally, flash EEPROMs are basically constituted of a memory cell array, a sense amplifier connected to a common input node of the memory cell array, a data input buffer also connected to the common input node and a write voltage setting circuit connected to the data input buffer.
However, the write voltage setting circuit has been so configured that the higher the temperature becomes, the lower an output voltage of the write voltage setting circuit. In addition, the higher the temperature becomes, the larger the equivalent resistance of a writing MOSFET becomes. As a result, if the temperature becomes high, a write voltage applied to a drain of a memory cell to be written becomes low. This has caused the following disadvantages:
(1) The higher the temperature of the memory cell to be written becomes, the longer a necessary writing time becomes. This becomes a problem in the memory characteristics.
(2) On the other hand, if the temperature of the memory cell to be written becomes lower, an excessive stress is applied on non-selected memory cells connected to the digit line to which a selected memory cell to be written is connected, and the written memory cells are erroneously erased. This is a problem in a reliability.
Therefore, the conventional semiconductor memory device has only a small temperature margin of the writing characteristics.